An Amateur Application of the Wien BridgeJanuary 1941 QST
Article

Wax nostalgic about and learn from the history of early electronics. See articles
from ARRL's QST, published December 1915 - present. All copyrights hereby acknowledged.

Wien bridge
oscillators had been around for a few years prior to this article in the January 1941 issue of QST magazine, but
evidently they had not been applied widely to amateur radio applications. In fact, Hewlett-Packard's very first product,
the Model 200A
audio oscillator, employed a Wien bridge oscillator as its frequency determining circuit. U.S. patent
2268872
provides a detailed description of its operation as well as the basic schematic. It was considered breakthrough design
because its amplitude and frequency stability rivaled that of the beat frequency oscillators of the day - at a small
fraction of the cost, size, and weight. The Model 200A quickly became a staple instrument on the benches of designers
and launched HP into the Keysight Technologies it is today (I'll forever wish HP had kept their
name for the Test & Measurement division and called their computers something else). The Hetrofil first
appeared in "Hetrofil - An Aid to Selectivity" in the
September 1939 QST.

An Amateur Application of the Wien Bridge

A. F. Oscillator for General Ham Use

By R. Wade Caywood* W1KRD

Fig. 1 - The fundamental Wien bridge circuit. Frequencies for which the bridge is balanced do not appear
in the output circuit.

Fig. 2 - A typical selectivity characteristic of the Wien bridge. This characteristic is basic in audio
oscillators using the circuit.

Fig. 3 - The equivalent parallel-T network, for use with grounded input and output circuits.

The above listed values are merely examples. Any resistors and condensers that satisfy the equations
may he used.

The Wien bridge is quite an old timer in the communication game, but it has remained in the laboratory until recently.
During the last year, however, there have been several commercial applications, although the "Hetrofil"1 is so
far the only amateur device using the principle.

The fundamental circuit, shown in Fig. 1, is quite simple, and with some modifications can be applied to such uses as
heterodyne reduction, oscillator frequency selection, sound analyzing, frequency measurement, capacity measurement, and
condenser power-factor measurement. Our present concern is with its application in simple audio oscillator circuits, of
the type suitable for speech amplifier testing and similar uses about the ham shack. Without going into the operation of
the bridge, already covered in Dr. Woodward's article.1 we may say that it is a selective network with a sharp
selectivity characteristic, Fig. 2 being typical.

In many applications of the bridge circuit it is necessary to use a transformer or a Wagner ground to get balance to
ground. The circuit in Fig. 3, the equivalent parallel-T network,2 has a common ground connection for input and
output, and is therefore frequently more convenient to use. The bridge shown in Fig. 1 can be tuned by varying the two resistors
R3 and R4 simultaneously, or by varying C1 and C2 simultaneously. The equivalent
parallel-T network can be tuned with three ganged resistors, R5, R6 and R7, or three ganged
condensers, C3, C4 and C5. If, in the circuit of Fig. 1, R1 =2R2,
C1 = C2, and the ganged resistors R3 and R4 have the same value, the simple
equation defining the frequency of the null point is:

Audio oscillators using the Wien bridge or its equivalent parallel-T network as a selective feed-back network have recently
been introduced commercially.3 Essentially such an oscillator is an amplifier with both positive and negative
feed-back. The positive feed-back occurs at all frequencies while the degenerative feed-back just cancels the positive feed-back
at all frequencies except the frequency for which the network is tuned. By varying this null point, the frequency of oscillation
is varied. An audio oscillator using the Wien bridge or its equivalent-T for the degenerative feed-back has many advantages
over the conventional heterodyne and LC audio oscillators. The heterodyne oscillator is a complicated affair with many tubes
and circuits; while the LC oscillator requires an iron-core coil and large condensers to tune to audio frequencies, with
the result that the frequency is usually varied in steps rather than continuously. The Wien bridge oscillator can be made
quite simply, requiring only one tube, and can be tuned continuously by resistances; in addition, it gives practically harmonic-free
output because of the sharp characteristic curve of the degenerative network.

Fig. 4-A is a simplified oscillator of this type, using the parallel-T network. The Wien bridge and a transformer are
used as the selective network in the oscillator shown in Fig. 4-B. A Hetrofil can be used as the bridge in the latter circuit.
In both circuits the positive feed-back is obtained through the 1:3 transformer, T1, with the low-impedance side
in the grid circuit. The amount of regeneration is controlled by the cathode resistor, R9. A by-pass condenser
across R9 is apt to cause the oscillator to produce all sorts of gurgling sounds, and therefore should not be
used. The oscillator can be tuned by either of the methods previously described. If the feed-back is too great a resistor,
R8, of the order of 500 ohms, will have to be put in series with the grid lead.

The circuit of Fig. 4-A has the advantage of requiring only one cheap transformer, but has the disadvantage that a three-gang
resistor that stays ganged is needed. Condenser tuning might be used instead, or a number of feed-back circuits could be
switched in or out for different fixed frequencies. The circuit of Fig. 4-B has the advantage of being easily tuned by a
two-gang resistor, but has the disadvantage of requiring two transformers.

It would be possible to obtain the 180-degree phase shift necessary for degeneration by using a voltage from the plate
circuit of the second section of a double-triode tube. This would make possible the elimination of a transformer, thereby
cutting cost and reducing size. However, in this connection it must be pointed out that the two feed-back voltages applied
to the grid must be exactly 180 degrees out of phase. This condition is hard to fulfill when more than one tube is used.

In Fig. 4-A, the resistance of the potentiometer across the output should be high so that the degenerative voltage is
unaffected. The resistors in the degenerative network should be fairly high in value so that a substantial degenerative
voltage can be developed at low current.

The cathode-resistor control should
be advanced only far enough to set up reliable oscillation. The plate-supply voltage can be varied from about 150 volts
to about 450 volts without causing a noticeable shift in oscillator frequency.

At medium and low output, the harmonic distortion is so small as to be negligible. However, at high output a small second
harmonic is discernible. The output volume is too great for comfort when using a headset, but if greater volume is needed
for loud-speaker operation it would be advisable to add an amplifier rather than to try to get more output directly from
the oscillator. This will keep the harmonic content to a minimum.

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